Taxol is a natural compound having the following chemical formula, which can be obtained from a trunk of Taxus brevifolia in small quantities.

It is known in the art that taxol has an antitumor activity and its action mechanism is based on the depolymerization inhibition action of microtubule during cell division, and thereby it was expected as an antitumor agent, which is different from the general antitumor agents in clinical application.
Although taxol can be only obtained naturally in extremely small quantities, recently the synthesis of taxol derivatives were disclosed by using a starting material 10-O-deacethylbaccatinIII having the following structure:

Compounds having the formula III as taxol precursors can be obtained from leaves and the like of taxaceous plants in relatively large quantities. Among the compounds, a compound (Taxotere™) having the following structure has drawn attention as a compound with an antitumor activity similar to or better than that of taxol and is now under development as an antitumor agent:

Taxol and Taxotere™ are also the desired antitumor compounds. However, according to the clinical tests, it is known that these compounds have low efficacy on digestive organ cancers, especially large bowel cancers, so that great concern has been directed toward the development of derivatives that have stronger antitumor effect.
The 9-position of taxol derivatives is generally a ketone group, but some derivatives are also known in which 9-position is reduced. A compound having a α-configuration hydroxyl group at 9-position was obtained from a natural source, and various types of the 9-position α-hydroxyl group derivatives obtained by chemical modification of the compound have been reported. Also, it is known in the art that a compound having a β-configuration hydroxyl group at the 9-position can be synthesized chemically by reducing 10-O-deacethylbaccatin III using a reducing agent, and various types of the 9-position β-hydroxyl group derivatives obtained by chemical modification of the compound have been reported (for example, see in WO 94/20088).
As a result of extensive investigation, the present inventors have found that the antitumor activity of the aforementioned 9-position β-hydroxyl group taxol derivatives sharply increases when the 9-position hydroxyl group and 10-position hydroxyl group thereof are converted into cyclic acetal. The present invention has therefore been accomplished according to the above.